DE 524 329 discloses a device for the slow rotation of a steam turbine shaft. The exciting machine of the current generator coupled to the steam turbine shaft is operated as a motor in operational intermissions, in order to drive the turbine shaft. Since operating the generator as a motor requires higher rotational speeds than is necessary for rotary operation in the operational intermissions, a speed reduction gear is inserted between the rotor shaft and the drive shaft of the exciting machine.
It is known, furthermore, that, in a rundown program after the shutdown of a gas turbine, the rotor mounted in an oil bearing is rotated at a lower rotational speed in rotary operation, as it is known, by means of a turning gear. The components, heated and expanded through the operation of the turbine, are thus cooled, during this cooling phase, from the operating temperature of the gas turbine to the ambient temperature. The compressor sucks in ambient air and pumps this into the annular flow path for the combustion chamber and turbine, so that the components are cooled and heat is extracted from the gas turbine.
The oil bearing is in this case fed not only from a lubricating oil supply, but additionally from a boosting oil supply which serves for boosting the rotor hydrostatically during rotary operation.
After the turning gear has been switched off, an air draft through the compressor, the combustion chamber and the turbine may occur, which is designated as natural draft and depends on the weather situation. This air draft may become so large that, during the rundown program, the rotor of the gas turbine continues to remain in rotational movement, despite the turning gear being switched off.
The disadvantage of this is that the control of the gas turbine carrying out the rundown program does not then automatically switch off the oil supply of the oil bearing on account of the continual rotational movement of the rotor. The automated switch off of the oil supply of the oil bearing would take place only when the sensor arrangement monitoring the rotational speed detects the standstill of the rotor. This results in fault warnings from the controls which then require manual action. To brake the rotor, it is then necessary to switch off the oil supply manually, the rotor then rotating, unlubricated in the oil bearing, until it comes to a standstill. This may lead to wear and defects on the rotor and oil bearing.